The present invention is generally directed to heat sink structures and to manufacturing processes for producing heat sinks having high-performance cooling capabilities. More particularly, the invention is directed to heat sinks having a large plurality of fin structures which exhibit a large surface area per unit volume. Even more particularly, the present invention is directed to high-performance millifin heat sinks having improved structural rigidity, excellent thermal transfer characteristics and ease of manufacture.
Since electronic circuit chips and other electronic circuit components are being designed with ever increasingly higher power ratings, the challenge of cooling these devices and systems becomes more difficult. The cooling challenge is seen to be directly related to the desire to have these circuit chip devices operate at increasingly higher frequencies. Accordingly, there is a growing desire and need to provide mechanisms and heat sink structures which are particularly effective and efficient for cooling these devices.
However, while it is known that it is desirable to increase cooling surface area, it is also known that there are penalties for employing structures with fins which have a high aspect ratio, that is, ratio of height to cross-sectional area. In particular, there is not only the problem of structural rigidity, but the actual manufacture of devices exhibiting very thin fins is also difficult. It is seen that heat sink devices with fins having a small cross-sectional area are subject to damage because of the fin length relative to its base area: the larger the fin height, the greater the problem; the smaller the base area, the greater the problem.
Furthermore, it is hard to manufacture heat sinks made from desirable thermally conductive materials which, nonetheless, exhibit long fin structures having a high surface area. Thus, while it is desirable to make heat sinks of materials such as copper and aluminum, the ability to produce heat sink structures of these materials is limited by the lack of manufacturing methods. The use of such materials in heat sinks exhibiting small length scales has proven to be difficult.
However, the desirability of increasing the surface area per volume is clearly seen by considering a heat sink having a regular array of in-line square "pin fins". To see this more easily, let us assume that the length of the heat sink base is L and that the height of each one of the fins is H. It is further assumed that there is a square pin fin at each corner of the heat sink and that there is a total of m spaces and fins along each edge. If this is the case, there are (m+1)/2 square pin fins along each edge of the square heat sink. It is noted that m is thus always odd (because there is a fin at each corner and because it takes two fins to define a space) and greater than 1. Thus, the regular array possesses N.sup.2 fins where N=(m+1)/2. Assuming that each pin and inter-pin space is the same along each edge, each pin has a side of length L/m. Furthermore, it is readily seen that each of these square cross-section pins possesses a perimeter of length 4 L/m and, thus, a total surface area of 4 LH/m. Therefore, the total surface area for all of the fins in the heat sink is 4 N.sup.2 LH/m, where N=(m+1)/2. For the case of m=21, that is, for the case in which the pin fins are arranged in an 11 by 11 array (N=11), one can compute the total pin surface area (excepting the top area of the pin); this number is referred to herein as A.sub.21. In a similar manner, for the case in which m=11 (that is, N=6), the heat sink has an array of 6 rows of pin fins with 6 pins in each row. In this case, one computes the total pin area, referred to herein as A.sub.11. For this particular configuration, the ratio A.sub.21 /A.sub.11 turns out to be equal to 1.76. This means that by increasing the number of pins from a 36-pin array to an array having 121 fins, there is a 76 percent increase in fin area. This increase in surface area occurs without any corresponding increase in heat sink volume.
Thus, it is seen while it is very desirable to increase the pin fin area, it is still nonetheless seen that the large number of fins desired together with their small size in terms of cross-sectional area makes it very difficult to manufacture these devices in an efficient yet economical fashion. In particular, it is noted that with an increasing value m, the number and complexity of machining operations goes to a point where it is not practical to construct such devices. This is particularly true when one wishes to construct heat sinks out of highly thermally conductive material such as copper. Likewise, one cannot easily extrude such heat sinks since their structures are two dimensional or at least include two dimensional aspects which cannot be fabricated using extrusion methods. Similarly, methods of manufacturing such devices using casting techniques are not practical because of the high aspect ratio associated with each of the (pin) fins.